1. Field of the Invention
The present invention relates to an antenna for radars. It can be applied notably to designation and trajectography radars wherein the designation function consists, for example, in keeping watch in a limited solid angle that makes it possible, in the event of detection, to give an approximate direction of this target. More generally, it can be applied to all radars one of whose functions requires precise localization of targets and swift reaction to instances of detection.
To attain high acuity, antennas need to be large-sized in order to obtain thin beams. Furthermore, high precision of detection calls for sufficient rigidity and stability of the mechanical structure of the antenna. Finally the requirement of short reaction times means that these antennas should rotate and take position swiftly. In the range of frequencies of the L band for example, these constraints become difficult to meet, notably because of the large sizes brought into play, as well as the weights of the antennas. In the latter case, the mechanical servo-control systems are particularly ill-suited to meeting the constraints of speed and mechanical precision.
Examples of antenna structures seeking to meet the above-mentioned constraints are known. These antennas are notably designed for target designation and trajectography radars. A known structure consists of a paraboloidal antenna, large-sized in relation to the wavelength, that can be swivelled by means of a turret. The designation function is obtained by the mechanical shifting of the reflector of the antenna, the axis of which explores the angular zone to be watched. However, in this case, the watch is sequential and hence slow. Furthermore, the making of the structures is difficult, notably because the volume and weight of the assembly are great and because the servomechanisms have to ensure that the positions are obtained with high precision.
Another known embodiment consists of the use of an electronic scanning antenna which, by means of its phase-shifters, can shift the axis of its beam speedily. The watching time is thus reduced, but this is a expensive approach, notably because of the large number of phase-shifters needed. Furthermore, in this case, it is difficult for the antenna to carry out a self-calibration. The absence of this possibility of self-calibration means that great care must be taken in the manufacture of such antennas, and this tends to further increase their cost.